Laminated electrophotographic unit and process

ABSTRACT

A unit adapted for use in electrophotographic cameras and an electrophotographic process are disclosed where the unit has electrostatic charges and comprises a photosensitive member consisting of a photoconductive layer and an electroconductive layer electrostatically attached to an insulating layer. If the unit is to be used as a film and stored for later exposure, it is sealed in an encasing sheet of the insulating material. If the unit is to be exposed directly, the insulating layer can comprise a master containing an image. During formation of the unit, the photoconductive layer is given a charge of a first polarity and the insulating layer is given a charge of an opposite polarity prior to contacting the two and prior to exposure to radiation. A preferred embodiment utilizes a clear plastic insulating layer with a resistivity of 1014 to 1018 ohms-meters and a dielectric constant of about 2 which in the form of a film contains no image and in the form of a printing element contains an image.

United States Patent [1 1 [111 3,798,029 Saklikar Mar. 19, 1974 [54]LAMINATED ELECTROPHOTOGRAPHIC 3,442,645 5/1969 Olden 96/1 R x UNIT ANDPROCESS 2,955.938 10/1960 Steinhilpen 96/L4 [75] Inventor: Arvind R.Saklikar, Park Forest, 111.

[73] Assignee: The Sherwin-Williams Company,

Cleveland, Ohio [22] Filed: July 27, 1971 [21] Appl. No.: 166,428

[52] U.S. Cl. 96/1 R, 96/1 C, 96/1.5,

[51] Int. Cl G03g 13/22 [58] Field of Search 96/1 R, l C, 1.5-1.8;250/495; 317/262 A; 355/17 [5 6] References Cited UNITED STATES PATENTS3,288,605 11/1966 Macklem 96/1 R 3,607,259 9/1971 Wright 96/1 C3,429,701 2/1969 Koehler l 96/1 R 2,833,930 5/1958 Walkup 250/4952 C3,322,538 5/1967 Redington.... 96/1 R 3,612,681 10/1971 ltoh 355/123,147,679 9/1964 Schaffert 96/1 R 3,582,731 6/1971 Sato et al 317/262 A3,650,623 3/1972 Beispel 355/12 Primary ExaminerCharles E. Van Horn 5 7]ABSTRACT A unit adapted for use in electrophotographic cameras and anelectrophotographic process are disclosed where the unit haselectrostatic charges and comprises a photosensitive member consistingof a photoconductive layer and an electroconductive layerelectrostatically attached to an insulating layer. If the unit is to beused as a film and stored for later exposure, it is sealed in anencasing sheet of the insulating material. If the unit is to be exposeddirectly, the insulating layer can comprise a master containing animage. During formation of the unit, the photoconductive layer is givena charge of a first polarity and the insulating layer is given a chargeof an opposite polarity prior to contacting the two and prior toexposure to radiation. A preferred embodiment utilizes a clear plasticinsulating layer with a resistivity of 10 to 10 ohms-meters and adielectric constant of about 2 which in the form 6f a film contains noimage and in the form of a printing element contains an image.

5 Claims, 3 Drawing Figures PATENTEDMAR 19 1974 3; 798029 NEGATIVE HlGHW VOLTAGE POWER SOURCE VOLTAGE POWER SOURCE INVENTOR ARVIND R. SAKLIKARBY M {2 4 ATTORNEYS LAMINATED ELECTROPHOTOGRAPHIC UNIT AND PROCESS Thepresent invention provides a method and means for electrophotographiccopying wherein the principles of the electrophotographic reproductionare improved and are applied in one embodiment to conventional camerasand in another to engineering template production.

In its broadest aspect, this invention comprises positively applying anelectrostatic charge to a photoconductive surface, as is typical inelectrostatic printing, and laminating this surface prior to exposure toan oppositely charged transparent insulating layer. One specificapplication of the invention provides a film adapted for long-termstorage which can be exposed to a projected image far from corona orlike charging apparatus. Another specific application provides animproved template for the construction industry produced by contactimaging without arc spots thereon.

Although it is recognized in electrophotography that oppositely chargedphotoconductive and insulating surfaces are held together byelectrostatic attraction, all known processes rely on charges on one orboth the layers applied or induced at the time of exposure. This meansexposures must be made in the vicinity of charging means. This practiceis caused by the tendency of charges to decay.

The use of electrophotographic principles in photography is also knownand it has been proposed to use film in such cameras which receives anelectrostatic image which then can be developed according toelectrophotographic methods. Such prior proposals are illustrated by US.Pat. No. 3,237,197; however, such proposals have suffered from thedeficiency of requiring complex mechanisms associated with the camerawith which to electrostatically charge the photosensitive film memberprior to imaging. Moreover, it is also found that the imaged film doesnot retain the image and must be developed and printed shortly afterexposure, in order to prevent charge decay resulting in faint printsfrom the film.

In the field of reproducing engineering drawings on conductivetemplates, the use of electrophotographic principles is also known;however, conventional methods of electrophotographic contact printingare used therein and the templates are frequently ruined or marred bysparking which results when the master is pulled away from the exposedconductive template. Moreover, it is discovered that a master used insuch electrophotographic reproductions tends to develop inducedelectrostatic charges of a polarity like that on the charged printingelement and upon separation after a few uses, arcing is so heavy that anew master must be used.

It is, accordingly, an object of this invention to provide a filmadapted for use in all electrophotographic imaging systems which doesnot require complex charging means contained in or associated with theimaging device and which will produce a high clarity print duringexposure.

This invention is particularly adapted for use in thoseelectrophotographic systems wherein the photosensitive elements, priorto use, are provided with an electrostatic charge which is dissipated onexposure to irradiation in selected areas leaving a charge image on thepaper.

According to this invention, a photosensitive member is given a chargeand then contacted with an oppositely charged insulating layer prior toexposure. The charges do not decay, since decay is strongly dependentupon the dielectric constant and resistivity of the atmosphere and thecharged material. It is known in certain humidity conditions decay isgreater than it is in other conditions and this is in conformity withtheoretical expectations. Charge patterns usually decay in anexponentialfashion with the characteristic decay time being given by the equation1' p6 where p is the time constant, p is the resistivity of the imagebearing member and e is the dielectric constant of the member, allquantities being expressed in MKS units. The available resistivities insuitable image bearing members have heretofore limited the length oftime during which it has been possible to retain an electrostatic image.This has been particularly true with photoconductive insulatingmaterials where it has generally been necessary to develop the chargepattern very soon after its formation. Ordinary insulating materials canbe obtained with resistivities higher than those of most photoconductiveinsulators, but here also the resistivity is often inadequate to supportthe charge for the desired length of time.

By contacting the charged photoconductor with an oppositely chargedinsulating layer, according to this invention, the initial charges areretained and the charges do not migrate through the respective layersbecause each is in contact with an attracting charge of an oppositepolarity. When used (e.g. exposed to light) portions of the charge onthe photoconductive surface are dissipated leaving an electrostaticimage pattern thereon but the remaining charges are protected fromdissipation by the insulating layer until developed. Thus, the unit ofthis invention will retain an initial charge prior to use and willretain a sharp image after exposure for months during storage, ifdesired. When it is desired to develop the image and the insulatinglayer is peeled off, it tries to induce a charge opposite to its own onthe photoconductor and the photoconductor acts likewise. This results ina decrease of electrical field between the two layers, so that no arcingresults and a clear unspotted image is reproduced on the photoconductivesurface.

The invention generally improves electrophotographic processes overthose wherein electrostatic attraction of an original and a copy sheetwas induced during exposure by increasing the speed and efficiency ofthe charging and exposing steps. Positive charging allows faster andmore uniform development of electrostatic charges. Moreover, it is foundthat the unit of this invention produces an electrostatic image fasterduring exposure because the electrostatic charge built upon thephotoconductor can dissipate in two directions whereas an unchargedmaster allows dissipation of the contacted photoconductor in only onedirection. This is of particular advantage when using the unit in acamera with short shutter opening times.

The invention will be better understood by reference to the followingFigures and specific Examples.

In the Figures, like numerals represent like elements and FIG. Irepresents a cross-section of an electrophotographic unit according tothis invention.

FIG. ll diagrammatically represents steps in the use and formation of aunit of FIG. I.

FIG. III represents projection imaging of a unit of FIG. I.

Referring more specifically to FIG. I, the electrophotographic unit isgenerally designated 1 and is seen to comprise a photosensitive membercomprising a photoconductive layer 3 with a first charge, shown asnegative and electroconductive layer 5 which is covered by an insulatinglayer 7 having an opposite charge, shown as positive The photoconductivelayer 3 may be any organic or inorganic photosensitive orphotoconductive substance typically used in electrophotography. Typicalsystems used for this purpose in the art are selenium, zinc oxide (aloneor dispersed in a binder) and certain organic non-polymeric materials,such as anthracene and anthraquinone, as well as organic polymericmaterials found to be photoconductive. These materials are normally notelectrically conductive but are activated on exposure to incidentradiation to an electroconductive state. In the art, inorganicmaterials, such as selenium and cadmium sulfide, are often vacuumdeposited on a base. Other inorganic materials, such as zinc oxide, andorganic materials, such as oxadizoles, pyrazolines, imidazoles,triazoles, etc.,- are applied with binders, such as polystyrene. Stillother photoconductive organic polymers are directly coated on the basefrom solutions, such as poly-N-vinylcarbazole, polyacenaphthylene, andpolyvinyl triphenylpyrazoline. Any of these and other systems can beutilized; however, the preferred element of this invention utilizes azinc oxide coating dispersed in a suitable insulating polymeric binder.This material is coated on a base material 5 which is also any of theconventionally used materials, such as paper, plastic or metal. If thematerial is paper or plastic, which are normally not electroconductive,it is typically provided with a sizing or coating (not shown) to renderit electroconductive. To improve adhesion of layer 3, it may bedesirable to provide a primer coat (not shown) on base 5 beforedepositing layer 3.

The backing material 5 may be either a flexible composition or may be arigid inflexible material, depending in part on the intended manner ofuse of the element. Examples of rigid materials which can be employedare, for instance, glass, unsaturated polyester resins; metals, such asaluminum, nickel, chromium, etc. (all in the form of plates, slides,disks, etc.). Metals are generally desired when the invention is to beused to produce engineering templates by contact imaging, as discussedbelow. Examples of flexible materials, which can advantageously beemployed as the backing material, are, for instance, polyethyleneterephthalate film sold by E. I. duPont de Nemours and Company ofWilmington, Delaware, under the name Mylar. A more refined grade ofpolyester terephthalic acid tape or film found highly appropriate as thebase for recording images is sold under the name ofCronar. Anotherflexible backing material which can be used advantageously because ofits good heat resistance, strength, inertness and resistance toradiation is polycarbonate resin film. Flexible backings are generallypreferred when the unit is to be used as camera film for projectionimaging, as discussed below.

The insulating layer 7 provided according to this invention should betransparent to the activating radiation. It is generally a sheet orfilm, preferably plastic, of any suitable thickness, preferably 0.5-mils. The

selection of a film is not based on the chemical nature of the film, buton the physical properties of the film. Thus, for use in this invention,the film should have a resistivity of from about IO to 10 ohm-meters anda dielectric constant of at least about 1.5. Specific films havingsuitable properties are found to include 7 mil thicknesses of Mylar filmand polyamide films, such as Nylon 66, as well as Teflon,polyacrylonitrile, and films from copolymers of vinylchloride and vinylacetate. Other materials, such as glass or metals, treated to have therequired insulating properties can be used.

Layer 7 may be applied as a temporary or permanent layer, againdepending primarily on the intended use. Ifa camera film is to beprepared, this film must generally withstand long storage times bothbefore and after exposure, so it is desirable to permanently apply layer7 to thereby seal and prevent atmospheric decay. In this case, the layer7 can be wrapped around the base containing the photoconductor or can beapplied only onto surface 3 and hermetically sealed in known manner. Forexample, a heated wire may be drawn across the overlapped ends or edgesof the film preferably while in a vacuum chamber to improve contact oflayers 7 and 3. The method of sealing the film forms no part of thisinvention. If a temporary layer is desired, for example, in a continuoustype process, such as shown in FIG. II, no sealing is required and theelectrostatic attraction of surfaces 3 and 7 will alone prevent chargedecay.

Whereas visible light is the preferred radiation used to form anelectrostatic image on the unit of this invention, other radiationcapable of activating layer (3) to an electroconductive state iscontemplated.

As one example of the scope of the invention, reference is made to itsapplication to the recording of X-ray patterns. In the case of X-rays,it will be recognized that metal layers are substantially transparent tosuch activating radiation, and thus the transpartent layer 7 may be ametal sheet or the like. Thus, specifically, it may be desired to forman electrical image from X-ray activation in which case both backingmember 5 and layer 7, or either of them, may be metal, and exposure tothe X-ray pattern may be accomplished at the desired stage in theoperations by exposure through such metal layer. In known manner, X-rayactivation is effective for forming a charge pattern on layer 3. It isto be understood, furthermore, that operation of the invention withX-rays or other penetrating radiation can be improved with a heavymetal, such as lead or the like, as a simulated intensifying screeneither as a nontransparent conductive electrode or backing member, or asa thin layer on the surface of the transparent insulating member 7through which the radiation sensitive layer is activated. Likewise, inview of the suitability of the invention for X-rays or penetratingradiation, the term photoconductive as applied to layer 3-should beinterpreted in its broader sense to denote a layer that is an insulatorin the absence of activating radiation and is rendered conductive by theaction of such radiation. It is already noted that the possibility ofX-ray activators also requires that the term transparent as applied tolayer 7 should be interpreted broadly to correspond to the intendedradiation.

According to this invention, the charge can be provided during formationof the films and it will not decay by reason of the protectiveinsulating layer. As shown in FIG. II, a previously preparedphotosensitive memher supply A and an insulating layer supply B areoppositely charged, for example, by exposure to corona discharge means23 and 25 and then contacted with the oppositely charged surfaces of thetwo laminae facing together. Illustrative contact means is shown asrollers 27. It is immaterial whether means 23 and 25 are positive ornegative respectively, so long as they supply opposite charges. Then atpoint 29 optional steps can be effected, depending on the intended use,e.g. the edges of the encasing film can be hermetically sealed by meansnot shown and e.g. the unit can be severed by means not shown. Thesystem illustrated in FIG. II is a continuous system where exposureoccurs immediately by radiation lamp 31. In this system, adapted formaking engineering templates, the insulating layer 7 will have beenprovided with an image. So long as the electrostatically attracted unitat point 29 is not contacted by activating radiation, it will retain itsinitial electrostatic charge for periods of up to six months.

As further shown in the continuous process of FIG. II, the unit isseparated after exposure by separating means 33, so that theelectrostatic image in layer 3 can be developed at developing station 35and fixed by suitable means (not shown) prior to storage at C. Theinsulating layer 7 is recovered as supply B for reuse. It is apparentthat layer 7 could be directly returned to supply B for recharging bycorona 23. Immediate separation 33 is not essential, as the image can beretained for months prior to developing.

If a non-continuous use is desired exposing means 31 and separatingmeans 33 can be by-passed and replaced at 29 by sealing and severingmeans to form a camera film.

When it is desired to use such a film, it can be placed in anyconventional camera, such as that shown as 9 in FIG. Ill and exposed toan image 19 through lens (13 and 15) and shutter 21 with focusing means17 and housing 11 provided as is typical in cameras. As already noted,the film may be exposed in use to light or X-ray or other activation toproduce an image 19.

The exposed film from FIG. Ill can be stored, out of contact withactivating radiation, and it can be developed by conventionalelectrostatic developing techniques by returning it to separating means33 and developer 35 of FIG. II. In developer 35 dry toner particles orliquid toner is applied to the surface 3 containing the electrostaticimage and the toner will be attracted and held to the surface 3whereupon the toner can be fused by conventional methods.

The liquid developers which can be employed can be of the type ascommonly known in the prior art. In addition, the following developerscan be used.

Positive developer. This developer is one containing developer materialwhich is attractable by negative electrostatic charges forming a latentelectrostatic image and thus this developer can be used to produce afacsimile or positive reproduction of an original on a charged andexposed electrophotographic sheet containing zinc oxide as thephotoconductor. The developer comprises a concentrate which is dispersedin the developer liquid to produce the liquid developer. The concentrateis prepared by grinding or milling lsol Ruby BKS red pigment 20 gramsBodied linseed oil 20 grams Lithographic varnish 20 grams Theconcentrate thus prepared can be dispersed in any developer liquidpossessing the required electrical properties, that is, avolumeresistivity of at least 10 ohm-cm. and a dielectric constant ofpreferably less than 3. The following liquids are suitable: n-hexane,nheptane, Shell X55, Shell X4, Shellsol T, Mineral Turpentine,cyclohexane, Solvesso 100, Freon 113, perchloroethylene and the like.

Negative developer. This developer contains developing material which isrepelled by negative electrostatic charges forming a latentelectrostatic image and which deposits onto areas free from such chargesor containing the minimum of such charges and thus, this developer canbe used to produce a negative or reversal reproduction of an original ona charged and exposed electrophotographic sheet containing zinc oxide asthe photoconductor. This developer is also sufficiently sensitive todevelop conductivity patterns on the photoconductive surface ashereinbefore referred to. The developer comprises a concentrate which isdispersed in the developer liquid to form the liquid developer. Theconcentrate is prepared by grinding or millmg Grams Carbon black pigmentl0 Automotive oil SAE 40 EXAMPLE I I A photoconductive coatingcomprising Grams Zinc Oxide (Durham Special 2) 600 Binder (RhodeneM8/50) 600 Solvent (Toluol) 250 Activators (4% Manganese Naphthenate 2.5and 3% Cobalt Naphthenate) 2.5

is prepared by milling the ingredients together, diluting and depositingon a relatively conducting support consisting of Bartya paper containingconductive carbon particles to produce a coating of about 1 milthickness.

This coated member and a 7 mil thickness of Mylar film were charged toabout 280 and 1200 volts respectively in a positive and negative Coronacharger by placing the respective members on grounded metal plates andsubjecting the exposed surfaces to about 6,000-l0,000 volts at lowamperage; (a negative charge was placed on the photoconductor and apositive charge on the Mylar).

The Mylar was then wrapped longitudinally around the electrophotographicmember and the edges overlapped while in a vacuum chamber with pressurereduced to about 1 mm Hg. Next, a heated tungsten filament was drawnacross the overlapped edges to seal the film and sever excess film. Asuitable length of film is cut, also in the vacuum chamber, by a hotknife which severs and seals the cut edges of the film.

Thisfil n was kept in the dark for abggtb months until placed in a boxcamera and exposed in normal manner. The exposed film was then removedand kept in the dark about 2 months until developed with the positivedeveloper listed above. The developer was fixed in normal manner andyielded a clear high quality positive reproduction.

EXAMPLE II A sheet of aluminum, five feet by ten feet and threeeighthsinch thick, was coated with about 0.5 mil epoxy resin primer.

This was then coated with a photosensitive composition comprising asolvent solution of zinc oxide particles, epoxy resin, and acrylicbinder (commercially known as CA 9251 of the Sherwin-Williams Company).This sheet was then passed under a corona and a negative electrostaticcharge of about 1,200 volts was produced on the photosensitive coating.

A Mylar sheet oflike dimensions which contained an inked engineeringdrawing was passed under a corona and a positive electrostatic charge ofabout 280 volts was produced thereon. The oppositely charged surfaces ofthe sheets were placed in contact by passing through driven rollers andthen exposed, separated, and developed, as in the prior Example, toyield a metal template free of spotting. The Mylar master was reusedcontinuously without development of spotting on the templates produced.

I claim:

1. An unexposed electrophotographic unit comprising a photosensitivemember comprising a photoconductive surface with an electroconductivebacking said photoconductive surface being sufficiently insulating toaccept and hold an electrostatic charge and said photoconductive surfacehaving an electrostatic charge of a first polarity by which it iselectrostatically attracted to an insulating transparent layer having anelectrostatic charge of the opposite polarity said insulating layerhaving a resistivity of from about l0 to about 10 ohm-meters and adielectric constant of at least about 1.5, wherein said insulating layeris free of conductive backing and external electrical connection.

2. The unit of claim 1 wherein the insulating layer is a plastic sheethaving a resistivity of about 10 ohm-m d a q cssnstE f abou 3. The unitof claim 1 wherein the insulating layer is hermetically sealed about thephotosensitive member.

4. In the method of electrophotographic reproduction wherein aphotoconductive surface is charged prior to exposure; the improvementwhich consists of charging a photoconductive layer, comprising aphotoconductive surface being sufficiently insulating to accept and holdan electrostatic charge, to a first polarity; charging a separatetransparent insulating member, having a resistivity of from about 10 toabout 10" ohm-meters and a dielectric constant of at least about 1.5 tothe opposite polarity; contacting the oppositely charged surfaces ofsaid members without substantial charge transfer; and then exposing theunit so formed to activating radiation.

5. The method of claim 4 wherein the insulating layer is a plastic sheethaving a volume conductivity of about 10 ohm-m and a dielectric constantof about 2.

2. The unit of claim 1 wherein the insulating layer is a plastic sheethaving a resistivity of about 1018 ohm-m and a dielectric constant ofabout
 2. 3. The unit of claim 1 wherein the insulating layer ishermetically sealed about the photosensitive member.
 4. In the method ofelectrophotographic reproduction wherein a photoconductive surface ischarged prior to exposure; the improvement which consists of charging aphotoconductive layer, comprising a photoconductive surface beingsufficiently insulating to accept and hold an electrostatic charge, to afirst polarity; charging a separate transparent insulating member,having a resistivity of from about 1014 to about 1018 ohm-meters and adielectric constant of at least about 1.5 to the opposite polarity;contacting the oppositely charged surfaces of said members withoutsubstantial charge transfer; and then exposing the unit so formed toactivating radiation.
 5. The method of claim 4 wherein the insulatinglayer is a plastic sheet having a volume conductivity of about 1018ohm-m and a dielectric constant of about 2.